Currently commercial windows and other fenestration systems mostly employ Aluminum framing because of the Aluminum Alloy's relatively low cost, high strength, easy manufacturability and long service life. However, Aluminum has one serious inherent disadvantage, which is high thermal conductivity. Traditionally, Aluminum framing has been plagued by poor thermal performance and low condensation resistance. There were some attempts to introduce pultruded fiberglass as a framing material for commercial framing, but these were abandoned due to very high cost and issues with manufacturing and durability. Steel reinforced PVC is also sometimes used, but this introduces thermal bridges, which largely defeat the benefits of the lower thermal conductivity of PVC. Also, there was no successful implementation of reinforced PVC in curtain walls and window walls, which represent large majority of commercial framing. Because of this, over the past couple of decades, the design of Aluminum framing has been modified to include thermal breaks of various designs. Technologies used for thermal break are generally divided into two categories: (a) Pour-and-debridge method, where the framing is extruded as a single piece with the pocket for thermal break. Liquid polyurethane is poured into the pocket and after solidifying, the backing Aluminum section is ground away. This is the older method, still in widespread use. The disadvantage of this method is that thermal break width is limited (typically it is about ¼ in.) by the structural requirements, and the thickness of the thermal break is fairly large, thus limiting the effectiveness of the thermal break. Windows incorporating this type of thermal break have generally a performance of about U=0.5 Btu/(hr·ft2·° F.), or R2. (b) Crimped strips (sometimes called I-bars), where frame is extruded into two dies and Polyamide strips (usually two) are crimped on each side to create single framing cross-section. Even though Polyamide has higher conductivity than Polyurethane, these strips have smaller cross-section (i.e., thinner) and can have larger widths than pour-and-debridge systems (normally around ½ in.), which allows for better frame performance (typically U=0.35 to 0.4 Btu/(hr·ft2·° F.) or up to R3). Their disadvantage is that this thermal performance cannot be easily improved further.
Additional methods consists of partial de-bridging of the framing web, or by using steel bolts at regular intervals to fasten indoor and outdoor frame sections.
While some of these methods have improved thermal performance of Aluminum framing, their relatively poor thermal performance still remains an issue and has resulted in relaxed code compliance requirements for commercial framing, as compared to residential framing. Namely, stricter structural requirements for commercial framing have prevented the use wood and PVC framing materials in commercial buildings, which are common materials in residential framing.